package once

import (
	"fmt"
	"strconv"
	"strings"
	"sync"
	"time"
)

func onceDo() {
	var once sync.Once

	f1 := print(1)
	once.Do(f1) // print

	f2 := print(5)
	once.Do(f2) // ignore
}

func print(i int) func() {
	return func() {
		fmt.Println("once:", i)
	}
}

// call is an in-flight or completed singleflight.Do call
type call struct {
	wg   sync.WaitGroup
	val  string
	rtt  time.Duration
	err  error
	dups int
}

// singleflight represents a class of work and forms a namespace in
// which units of work can be executed with duplicate suppression.
type singleflight struct {
	sync.Mutex                  // protects m
	m          map[string]*call // lazily initialized
}

// Do executes and returns the results of the given function, making
// sure that only one execution is in-flight for a given key at a
// time. If a duplicate comes in, the duplicate caller waits for the
// original to complete and receives the same results.
// The return value shared indicates whether v was given to multiple callers.
func (g *singleflight) Do(key string, fn func() (string, time.Duration, error)) (v string, rtt time.Duration, err error, shared bool) {
	g.Lock()
	if g.m == nil {
		g.m = make(map[string]*call)
	}
	if c, ok := g.m[key]; ok {
		c.dups++
		g.Unlock()
		c.wg.Wait()
		return c.val, c.rtt, c.err, true
	}
	c := new(call)
	c.wg.Add(1)
	g.m[key] = c
	g.Unlock()

	c.val, c.rtt, c.err = fn()
	c.wg.Done()

	g.Lock()
	delete(g.m, key)
	g.Unlock()

	return c.val, c.rtt, c.err, c.dups > 0
}

// 解决并发访问相同的参数请求
func concurrent(count int) strings.Builder {

	var res strings.Builder
	var w sync.WaitGroup
	w.Add(count)

	var s singleflight
	fn := func() (string, time.Duration, error) {
		return strconv.Itoa(count), 5 * time.Millisecond, nil
	}

	go func() {
		for i := 0; i < count/2; i++ {
			go func() {
				r, t, err, b := s.Do("test", fn)
				log := fmt.Sprintf("r:%v\tt:%v\terr:%v\tb:%v\n", r, t, err, b)
				res.WriteString(log)
				w.Done()
			}()
		}
	}()

	go func() {
		for i := count / 2; i < count; i++ {
			go func() {
				r, t, err, b := s.Do("test", fn)
				log := fmt.Sprintf("r:%v\tt:%v\terr:%v\tb:%v\n", r, t, err, b)
				res.WriteString(log)
				w.Done()
			}()
		}
	}()

	w.Wait()

	return res
}
